I know it is AMD, but the actual cards generally have another name on them as well.
The reason I ask this is because as I said, they only made a small handful of 1gigabyte 4870 cards. You may well just have a 512 megabyte card.
GPU-z will tell you once and for all EXACTLY what you have on the card.
http://www.techpowerup.com/downloads/SysInfo/GPU-Z/